Polarization properties of retroreflecting right-angle prisms

The cumulative retardance t introduced between the p and the s orthogonal linear polarizations after
two successive total internal reflections (TIRs) inside a right-angle prism at complementary angles and
90° is calculated as a function of and prism refractive index n. Quarter-wave retardation (QWR)
is obtained on retroreflection with minimum angular sensitivity when n 2 112 1.55377 and
45°. A QWR prism made of N-BAK4 Schott glass (n 1.55377 at 1303.5 nm) has good spectral
response (5° retardance error) over the 0.5–2 m visible and near-IR spectral range. A ZnS-coated
right-angle Si prism achieves QWR with an error of 2.5° in the 9–11 m (CO2 laser) IR spectral range.
This device functions as a linear-to-circular polarization transformer and can be tuned to exact QWR at
any desired wavelength (within a given range) by tilting the prism by a small angle around 45°. A
PbTe right-angle prism introduces near-half-wave retardation (near-HWR) with a
2% error over a broad 4 12.5 m IR spectral range. This device also has a wide field of view and its interesting
polarization properties are discussed. A compact (aspect ratio of 2), in-line, HWR is described that uses
a chevron dual Fresnel rhomb with four TIRs at the same angle 45°. Finally, a useful algorithm is
presented that transforms a three-term Sellmeier dispersion relation of a transparent optical material to
an equivalent cubic equation that can be solved for the wavelengths at which the refractive index assumes
any desired value.